Except a few low-voltage and low-power electronic apparatuses, the majority of batteries are connected in series in use, because the voltage of a single battery cell is not high enough, so its power loss is greater when it is used alone. For example, under the same load power, the voltage and capacity for a system of four batteries connected in series are four times as powerful as that of a single battery. However, the required output current is only one-fourth of that required for the single battery system. In this way, transmission loss is reduced by 16 times. Although there are advantages inherent in applying serially connected battery strings, practically speaking, its overall efficiency is less than expected, if the serially connected battery strings are only considered as a “single battery cell” having high voltage and great capacity, without taking the differences among batteries therein into account.
Properties such as recharge-discharge capacity, recharge-discharge transfer efficiency, initial voltage, and internal resistance of secondary batteries are hardly the same, even if they come from the same batch. Therefore, if these single batteries comprising minor property differences are connected in series, the user will gradually find the life of the battery power source becomes shorter, with an increase in repeated recharges and discharges, although such property differences can hardly be recognized in the very beginning. Therefore, the batteries are frequently required to be recharged. In addition, during recharging and discharging the batteries, the temperature of the serially connected battery strings are found to be much higher than that of new batteries, because when the serially connected battery string is first used, differences among single batteries are not great. However, with more recharges and discharges, differences among these single batteries become greater. In this vicious cycle, poor performance single batteries will become even worse, thereby making the performance of the serially connected battery strings being restricted by these poor performance single batteries. Therefore, to solve the above-mentioned problems, it is necessary to monitor single batteries in the serially connected battery strings, and then use battery voltage equalizers to equalize the stored energy imbalance caused by differences in single batteries, thereby increasing the actual rechargeable and dischargeable capacity of the serially connected battery strings, and thus extend their service life.
The size of voltage of the batteries in the serially connected battery strings is related to the capacity and the charge amount stored in the batteries. To equalize the voltage of each battery, energy transfer is required. In other words, it is to dissipate the energy of higher-voltage batteries or transfer the energy of higher-voltage batteries to lower-voltage batteries. On the basis of energy dissipation, battery voltage equalizers can be classified into two types: dissipative type and non-dissipative type. The former converts the energy stored in the higher-voltage batteries into thermal energy by means of switching resistive load, or to equalize voltages by means of switching buffer capacitor. The latter transfers energy in the batteries by means of switching direct current (DC) converters, if switching loss is ignored, theoretically speaking, no-loss transmission can be achieved. On the basis of energy transfer for the non-dissipative type battery voltage equalizer, this can be further classified into two types: total charge distribution and single charge distribution. The former is to absorb or provide energy for the total serially connected batteries, in order to equalize the voltages of individual batteries, whereas the latter achieves equalization through the energy transfer among adjacent single battery cells. On one hand, if the power converters for the total charge distribution are mutually independent, it is called a distributed battery voltage-equalizing device; On the other hand, if a single power converter is utilized, then it is called a centralized battery voltage-equalizing device.
According to prior technology, FIG. 1 shows a circuit structural view of a dissipative type battery voltage-equalizing device, in which, through the actions of the switch, the energy of higher-battery-end-voltage battery 3 is dissipated on the load resistance of individual voltage equalizers 2 thereon. Although the circuit structure is simple, it is necessary to detect individual battery-end-voltages. Additionally, this also involves the issue of heat dissipation. As for non-dissipative-type battery voltage equalization technology, FIG. 2 depicts a circuit structural view of a battery voltage-equalizing device based on single charge distribution, in which two adjacent battery cells serve as equalization mechanisms by transferring energy from a higher-battery-end-voltage battery to a lower-battery-end-voltage battery. In addition, the circuit structure has modular extensibility. FIG. 3 shows a circuit structural view of a distributed battery voltage-equalizing device based on total charge distribution, in which all DC converter circuits are mutually independent. When the battery voltage is different from the mean value, DC converter circuits are started, so as to release excessive battery energy and then transfer it to the serially connected battery strings or transfer the extra energy from the serially connected battery strings to the lower-voltage battery 3. Therefore, it is highly controllable. FIG. 4 shows a circuit structural view of a centralized battery voltage-equalizing device based on total charge distribution, in which a single DC converter acts as the voltage equalizer for individual batteries in the serially connected battery strings. Theoretically speaking, this centralized battery voltage equalization circuit is small and has reduced cost.